The present invention relates to adaptive optics and, more particularly, to an optical electric-field pattern generator using a pair of phase spatial light modulators to respectively modulate the amplitude and phase of an input laser beam.
Lasers are advantageously used in free space communications because they can transmit substantially more information quite rapidly over long distances in comparison to radio frequency waves or microwaves, primarily because optical waves have a much higher frequency than the foregoing waves and thus provide a significantly greater bandwidth. The problem inherent to free space communications, in contradistinction to transmitting light through an optical fiber, is that when some portion of the propagation path passes through the relatively dense atmosphere surrounding the earth, the beam is affected by atmospheric turbulence. More particularly, the atmospheric density will not remain constant transversely across the beam cross section, and it will also change along the propagation path as a function of time.
Time-varying nonuniformities in atmospheric conditions along the path of the laser beam affect it in several ways. Firstly, a change in density due to turbulence or temperature alters the index of refraction, with several consequential effects. As the paths of individual rays composing the beam will usually be affected differently due to the heterogeneous nature of the turbulence, the respective path lengths will be unequal and result in differing phases and thus interference. This causes changes in the amplitude of the laser beam across the receiver aperture. As a result, the received laser beam will no longer have either the uniform amplitude or phase that it had when emitted.
Furthermore, the random change in direction of the rays composing the laser beam increases the lateral cross section of the beam. This decreases the portion of the beam impinging the receiver aperture, in comparison to a transmission path without turbulence. As a consequence, the received optical power is reduced, resulting in a decrease in the signal to noise ratio.
The prior art resolves the foregoing difficulties encountered in atmospheric laser transmissions by first sensing the conditions present along the intended path. This is typically accomplished by determining the changes incurred by a beacon beam transmitted along the intended path immediately before the communication beam is sent. The appropriate corrections for the phase and amplitude to be applied to the communication beam are then determined using algorithms well known in the art. Lastly, the appropriate corrective amplitude and phase are applied to the transmitted communication beam such that, after traversing the same path as the beacon beam in the reverse direction and being distorted en route by atmospheric turbulence, the beam arriving at the receiver is composed of the amplitude and phase that it would have had in the absence of turbulence. The apparatus that applies the corrective spatial pattern of phase and amplitude is called an optical electric-field pattern generator.
The corrective pattern, also known as a conjugate pattern, is applied to a laser beam having a uniform amplitude and phase. The conjugate pattern has an intensity profile identical to that of the transmitted beacon beam and a phase profile that is the inverse of that of the distorted beacon beam. The conjugate pattern is typically obtained by successively applying the prescribed amplitude (the square root of the intensity) and phase modulations. The amplitude modulation is applied using a liquid crystal or semiconductor device, and the desired phase modulation is obtained with a phase spatial light modulator.
Both of the aforementioned devices commonly used for amplitude modulation have acknowledged limitations. The liquid crystal devices have limited speed. The semiconductor devices have a limited range of wavelengths over which they operate, also known as a spectral acceptance width. Furthermore, most of the semiconductor devices modulate amplitude by absorbing light, and thus generate heat that requires disposal.
Lasers are also employed in conjunction with high speed programmable masks for optical pattern recognition to quickly determine whether a known pattern matches one of a great number of patterns, e. g., fingerprints. The aforementioned shortcoming in the speed with which liquid crystal devices can modulate amplitude, directly limits how quickly a pattern recognition task can be completed.
As may be seen from the foregoing, there presently exists a need in the art for an optical electric-field pattern generator that modulates both the amplitude and phase of a laser beam at high speed, with a high spectral acceptance width, and without generating heat. The present invention fulfills this need in the art.
Briefly, the present invention is an apparatus that modulates the amplitude and phase of a laser beam using two spatial light modulators in combination with conventional passive components. A first spatial light modulator, a high reflectivity mirror and a 50/50 light beamsplitter combine to form a two-dimensional array of Michelson interferometers that modulate the amplitude of an input laser beam through using constructive and destructive interference.
The amplitude pattern of the modulated output beam is varied by mechanically adjusting the path length difference between the two interferometer arms. An electronic feedback system maintains a desired amplitude profile by monitoring the output intensity of one or a small number of nonactivated, stationary reference pixels on the first spatial light modulator. Such a system will automatically compensate for drift caused by the heating or vibration of components and structure.
The amplitude-modulated beam is subsequently directed successively through an electric-field imaging telescope, a polarization beamsplitter, and a quarter-wave plate, before impinging the reflective surface of a second spatial light modulator. The second spatial light modulator is adjusted to provide the desired phase profile of the output beam. The beam is then again directed through the quarter-wave plate and, as its plane of vibration has been rotated 90xc2x0 by two transits through the quarter-wave plate, it is subsequently reflected off of the polarization beamsplitter and out of the apparatus.
The apparatus of the present invention uses reflection to modulate both the amplitude and the phase of a laser beam. It can apply the desired modulations at a high speed, and can be used over a wide wavelength spectrum. Due to the nature of light reflection, light is not absorbed and minimal waste heat is generated.